In the film industry, film products are wound onto tubular plastic cores for handling and storage of the products. It is desirable to form the plastic cores from olefins, such as polypropylene, because of its relatively low cost. The plastic cores are made from tubing in a continuous extrusion process. The continuously extruded plastic core, however, must be cut into lengths to form cores of a suitable length. Inward pressure applied on the extruded plastic tubing during the cutting process results in localized deformation, or neck-down, of the tube adjacent to the cut ends of the cores. The neck-down involves inward deflection of the tube from its nominal dimensions that occurs without change in the thickness of the tube. The neck-down is particularly pronounced when the tube is cut without a mandrel inside the tube. Even when an inner mandrel is present during the cutting, however, some neck-down will still occur, most likely resulting from stress relief acting on the cut end.
Driven end plugs received in opposite ends of the tubular core may provide for winding rotation the tubular core. Alternatively, rotation may be provided by a mandrel received within the interior of the core. Torque is transferred to the core through frictional forces developed between the inner surface of the core and the end plugs or mandrel. To provide for an engagement between the drive members and the core that results in suitable friction, the dimensions of the respective engaging surfaces have close tolerances. The neck-down of the core ends associated with cutting, however, causes an inward deflection of the core inner surface. Such change in the dimensions of the inner surface detrimentally affects the engagement between the core and the drive member frequently rendering the core unusable. As discussed above, the neck-down of the core ends also results in inward deflection of the outer surface of the core on which the film product is wound. Non-uniformity in the outer dimensions of the core undesirably causes stretching of the film during winding of the film onto the necked-down core.
It is known to bell the ends of pipe made from plastic, such as polyvinyl chloride, to provide for end-to-end connection of transportable lengths of the pipe to form a conduit for conveying fluids or for containing electrical wiring for example. As described in U.S. Pat. No. 4,008,028 to Ronden et al., the pipes are belled by heating the end of the plastic tube to the softening temperature of the plastic and outwardly expanding the end of the pipe from its nominal dimensions against a shaping mandrel. In Ronden, lengths of the pipe to be belled are transported along a conveyor to heating and belling stations. Heating and belling heads are moveably mounted at the respective stations for advancement and retraction of the heads with respect to the conveyor. The movement of the heads provides for engagement of the heads with ends of the pipe.
The outward deformation of the pipe resulting from the belling process, however, causes thinning of the pipe wall from its nominal dimensions. In U.S. Pat. No. 4,406,852 to Riegel, the end of the pipe is thickened from its nominal dimensions to provide for the thinning associated with the belling process. To thicken the end of the pipe, the pipe is heated to its softening temperature and axially driven into an annular chamber having the desired dimensions for the thickened portion.
In known belling processes, the outwardly deformed pipe end is sometimes cooled prior to its separation from the shaping mandrel. In U.S. Pat. Nos. 4,008,028 and 4,083,918 (both to Ronden et al.), for example, a cooling ring is provided at the belling station to direct cooled air onto the pipe end. In U.S. Pat. No. 4,059,379 to Korff et al., the mandrel includes internal passages for circulation of either a heated fluid to facilitate the belling process or a fluid to facilitate cooling of the pipe before its removal from the mandrel. The location of the cooling means at the belling station in the Ronden and Korff devices, however, provides for an inefficient system in terms of heat transfer. To facilitate the outward deformation of the pipe, the pipe end must be maintained in a heat softened condition during the belling process. Therefore, the cooling means located at the belling station in Ronden and Korff cannot be active during the belling process and must be activated periodically after belling has been completed. As a result, the heat that is required for maintaining the pipe end in a heat softened condition for belling must be repeatedly added and then removed during each process cycle.